photosynthesis

photosynthesis defined in 1951 year

photosynthesis - photosynthesis;photosynthesis - In green plants, synthesis of organic compounds from water and carbon dioxide using energy absorbed by chlorophyll from sunlight. Source of most of the oxygen in the atmosphere. Takes place within chloroplasts. Basic for existence of virtually all other forms of life (exceptions being photosynthetic and chemosynthetic bacteria) since products form raw materials on which, directly or indirectly, all plants and animals depend for provision of energy for all the reactions that constitute metabolism. Photosynthesis may be summarized by the empirical equation CO2 + 2H2O -(Light energy + Green plant)-> (CH3O) + O2 + H2OAbsorption of light energy initiates a series of light (photochemical) reactions, a unique feature of photosynthesis. Oxygen is released from oxidation of water; a reducing agent NADPH2 is generated; light energy is converted in photophosphorylation to chemical energy in the form of the energy-rich compound ATP. These reactions take place in the grana of chloroplasts. They are followed, in the grana, by chemical reactions described, in contrast to photochemical reactions, as 'dark' reactions. Using the reducing power of NADPH2 and the energy of ATP, CO2 is reduced to carbohydrate (reductive CO2 fixation). Two pathways are known. In some plants CO2 is combined in the Calvin (its discoverer) cycle with a 5-carbon compound, ribulose diphosphate, which subsequently gives rise to two molecules of the 3-carbon compound phosphoglyceric acid. This in turn gives rise to phosphoglycer-aldehyde, triosephosphate, hexose phosphate and hexose. From phosphoglyceric acid, fatty acids and amino acids may be subsequently derived. ADP, NADP and phosphate are released for reincorporation into the cycle. In other plants the pathway to phosphoglyceric acid involves combination of carbon dioxide with the 3-carbon pyruvic acid to form the 4-carbon dicarboxylic acid oxaloacetic acid (with rapid interconversions with malic and aspartic acids). A carboxyl (COOH) group is transferred from oxaloacetic acid to an acceptor (possibly a 2 or 5-carbon sugar phosphate) forming phosphoglyceric acid. Thereafter hexose is formed by a series of reactions similar to those of the Calvin pathway.

Photosynthetic bacteria differ conspicuously from green plants in the source of hydrogen used in photosynthesis, being unable to use water and hence unable to produce oxygen. Most, in fact, are anaerobic. In the purple and green sulphur bacteria, hydrogen is derived from hydrogen sulphide and sulphur is a by-product. In the purple non-sulphur bacteria, organic compounds are used as hydrogen sources under anaerobic conditions, but some species, growing in the presence of oxygen and in darkness, use organic compounds as food directly, i.e. they behave as heterotrophs. See also: Chloroplast, Chlorophyll.